Fluid loss control

ABSTRACT

The invention, in one embodiment, relates to a fluid loss control additive or composition comprising a granular starch composition and fine particulate mica, in specified proportions. The invention further comprises a fracturing fluid containing a starch composition and mica, in a specified ratio. In yet a third embodiment, the invention comprises a method of fracturing a subterranean formation penetrated by a borehole, comprising injecting into the borehole and into contact with the formation, at a rate and pressure sufficient to fracture the formation, a fracturing fluid containing starch and mica, in specified ratios, and in an amount sufficient to provide fluid loss control. In an additional embodiment, a fluid loss additive is used in a low viscosity preliminary sacrificial conditioning stage solution which will condition high permeability formations to provide a substantially uniform, low permeability fracture face leading to longer fracture lengths and more predictable design and execution of fracturing treatments in high permeability formations.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of co-pending applicationSer. No. 281,786 filed on Jul. 28, 1994 now abandoned.

FIELD OF THE INVENTION

This invention relates to the recovery of hydrocarbon fluids fromsubterranean formations. More particularly, the invention relates to anovel fluid loss control additive combination for use in fracturingfluids, to a novel fracturing fluid containing such additivecombination, and to fracturing processes utilizing the novel fracturingfluid.

BACKGROUND OF THE INVENTION

In the recovery of hydrocarbon values from subterranean formations, ithas been common practice, particularly in formations of lowpermeability, to fracture the hydrocarbon-bearing formation to provideflow channels to facilitate production of the hydrocarbons to thewellbore. In such fracturing operations, a fracturing fluid ishydraulically injected down a well penetrating the subterraneanformation and is forced against the formation by pressure. Through thisprocedure, the formation is forced to crack or fracture, and a proppantis placed in the fracture. The fracture provides a radially oriented,relatively high permeability channel in the formation offering improvedflow of the recoverable fluid, i.e., oil, gas, or water, back into thewellbore. While a wide variety of fracturing fluids have been used,fracturing fluids customarily comprise a thickned or gelled aqueouscarrier fluid which has suspended therein "proppant" particles which aresubstantially insoluble in the carrier fluid and the fluids of theformation. Proppant particles carried by the fracturing fluid remain inthe fracture created, thus propping open the fracture when thefracturing pressure is released and the well is put into production.Suitable proppant materials include, but are not limited to, sand,walnut shells, sintered bauxite, or similar materials. As will beunderstood by those skilled in the art, the "propped" fracture providesa larger, more highly permeable flow channel to the wellbore throughwhich an increased quantity of hydrocarbons can flow, thereby increasingthe production rate of a well.

A problem common to many hydraulic fracturing operations is the loss offracturing fluid into the porous matrix of the formation, particularlyin formations of high permeability, e.g., formations having apermeability of greater than 2 md. Fracturing fluid loss isobjectionable, not only because of cost considerations, but especiallybecause it limits the fracture geometry which can be created in highpermeability formations. In general, fracturing fluid loss depends onthe properties of the rock in the formation, the properties of thefracturing fluid, the shear rate in the fracture, and the pressuredifference between the fluid injected and the pore pressure of the rockmatrix. In this regard, the properties of the fracturing fluid are thoseexhibited by the fluid in the formation as influenced, inter alia, bythe temperature and shear history to which the fluid has been subjectedin its travel down the wellbore and through the fracture.

Thorough analysis of the problem of fracturing fluid loss in highpermeability formations reveals that it is necessary to reduce "spurt".As used herein, the term "spurt" refers generally to the volume of fluidlost during fracturing because of early leak off of fracturing fluidbefore pores of the formation can be plugged, and/or before an externalfiltercake is formed on the newly exposed rock surface. In the past, avariety of additives to the fluid have been employed, most beingselected or designed to generate an external low-permeability filtercakequickly, under little or no shear stress (usually referred to as staticconditions) in order to cover the pores and stop spurt. This approach isunsatisfactory since high shear stresses eliminate or severely limit theformation of external filtercake.

In general, the higher the permeability of a rock, the greater the fluidlosses due to spurt are likely to be. However, it has been determinedthat during hydraulic fracturing, spurt occurs principally at or nearthe advancing tip of the fracture, where new rock surface is beinggenerated. The shear stresses that the fracturing fluid exerts on thesurface of the rock are greater proximate the tip of the fracturebecause of the narrower fracture gap in that location. As indicatedpreviously, the high shear stresses prevent the formation of externalfiltercakes of polymer and/or fluid loss additives by eroding thesurface of the cake in contact with the fracturing fluid. Accordingly,to be effective, a fluid loss additive must be able to stop spurt underhigh shear rates.

In fracturing high permeability formations, it is desired to developfractures which are wide and high but which do not extend the greatradial distance away from the wellbore as is common in the fracturing ofrelatively low permeability formations. In the design of highpermeability formation fracturing, it is desirable to achieve acondition called tip screen-out after the fracture has opened thedesired, relatively short, radial distance from the wellbore. Tip screenouts are achieved by allowing leakoff of the fracturing fluid into theformation to the point where there is insufficient fluid to suspend theassociated proppant which the fluid is carrying. In order to accuratelydesign such treatments for high permeability formations (greater than 50milidarcies), it has been necessary to know the precise permeability ofthe formation so that tip screen out will occur at the desired point inthe pumping schedule. If the formation permeability is not known or isonly estimated by calculating the permeability of adjacent wells orsimilar formations, any variation in the actual permeability of theformation being treated will cause the fracturing treatment to deviatefrom that designed since the desired tip screen out may occur too early,too late or not at all, therefore not returning full value for thetreatment performed. It would, thus, be desirable to be able to moreaccurately predict the formation permeability in high permeabilityformations or to temporarily alter the formation permeability to anaccurately predictable value prior to or during the fracturing treatmentso that the fracturing treatment can be executed in accordance with thedesired design. The present invention addresses this desirable result.

Williamson et al., (U.S. Pat. No. 4,997,581) describe the prior artutilization of a variety of inorganic solids, natural starches, andcombinations of finely divided inorganic solids with natural starches.All of these compositions are deemed by these patentees to beinsufficient to control fracturing fluid loss in moderate to highpermeability formations. While these patentees attempt to provide aneffective additive by the use of blends of natural starches and modifiedstarches, their blends have limited application. For example, forformations having high permeability and high temperatures, e.g., 300°F., natural and modified starches may not effectively plug the pores inthe fracture walls. Finally, additives suggested by other workers in theart, while providing some fluid loss control, often are prohibitive incost.

Accordingly, there has existed a need for a low cost additive orfracturing fluid which provides fracturing fluid loss control, and amethod of fracturing a subterranean formation characterized by reducedfluid loss, under a variety of conditions which include highpermeability, high shear rates and high temperature. The inventionanswers this need.

SUMMARY OF THE INVENTION

The present invention provides a relatively low cost, highly effectivefluid loss additive which is capable of being used under widely varyingconditions of bottom hole temperature, pressure and formationpermeability and to achieve the desired predictable tip screen outs informations of high but not accurately known permeability.

In accordance with the invention, one embodiment relates to a fluid losscontrol additive or composition comprising a granular starch compositionand fine particulate mica combined in a specified ratio or proportion.Preferably, the fluid loss control composition of the invention containsan additional finely divided inorganic solid, or mixture of such solids.The additive components of the invention may be added directly to asuitable fracturing fluid, or, for ease of formulation, the additivecomponents of the invention may be suspended in a suitable diluent orcarrier liquid, the fluid loss control additive-carrier liquidcombination then being combined with the fracturing fluid.

Further in accordance with the invention, a fracturing fluid comprises agranular starch composition and fine particulate mica, in a specifiedratio, and preferably contains the additional inorganic solid(s)mentioned, these components being supplied in the fracturing fluid inamount and proportion sufficient to provide fluid loss control.

Still further in accordance with the invention, a method of fracturing asubterranean formation penetrated by a borehole comprises injecting intothe borehole and into contact with the formation, at a rate and pressuresufficient to fracture the formation, a fracturing fluid compositioncontaining a granular starch composition and finely divided micacombined in a ratio specified more fully hereinafter and in an amountsufficient to provide fracturing fluid loss control. Preferably, thefracturing fluid used also contains a finely divided inorganic solid orsolids.

Still further in accordance with the invention, a method of fracturing ahigh permeability formation comprises providing a low-viscosity,sacrificial fracturing fluid containing a relatively large amount offluid loss additive and pumping this sacrificial fracturing fluid atfracturing pressure into a subterranean formation to close off formationpores and provide a relatively uniform, low and predictable permeabilityto the otherwise high permeability formation prior to pumping fracturingfluid pad and proppant carrying stages at fracture-extending pressure.

Still further in accordance with the invention, the above low-viscositysacrificial fracturing fluid includes a fluid loss additive comprising agranular starch composition and finely divided mica combined in a ratiospecified more fully hereinafter and in an amount sufficient to providefracturing fluid loss control and predictable resultant permeability tothe formation. Preferably, the sacrificial fracturing fluid used alsocontains a finely divided inorganic solid or solids.

It is therefore an object of this invention to provide a fracturingfluid and a fluid loss additive composition which is of relatively lowcost.

It is another object of this invention to provide a fracturing fluid anda fluid loss additive which may be used in a wide variety of formationconditions of temperature and permeability.

It is a further object of this invention to provide a low viscosity,sacrificial fracturing fluid which can be pumped prior to the pumping oftypical fracturing fluid stages and which results in the temporaryplugging of high permeability formation pores to achieve a relativelyuniform, predictable and reproducible apparent permeability which allowsmore precise execution of tip screen out fracture designs for such highpermeability formations.

DETAILED DESCRIPTION OF THE INVENTION

Any suitable granular starch or mixture of starches may be used in theinvention. Accordingly, as used hereinafter, the term "starchcomposition" is understood to include one or more natural starches, oneor more chemically modified starches, and mixtures of one or morenatural and/or chemically modified starches. Natural starches which maybe employed in the invention include, but are not limited to, those ofpotato, wheat, tapioca, rice, and corn, the preferred starch beingpotato starch.

Most preferably, pre-gelatinized starches, particularly pre-gelatinizedpotato starch, are employed. Pre-gelatinized starches may be obtainedcommercially or they may be prepared by pre-gelatinization treatment.For pre-gelatinization, the chosen starch granules are heated in waterto a point where the starch granules swell irreversibly. Upon cooling,this swollen structure is retained. The use of pre-gelatinized starchesyields an important advantage to the combination of the invention, sincethese materials are stable at higher temperatures in the formation,e.g., up to 300° F.

Chemically modified starches are those derived from natural starches bychemical reaction of a natural starch with a suitable organic reactant.Chemically modified starches which may be used in the invention include,but are not limited to, carboxymethyl starch, hydroxyethyl starch,hydroxypropyl starch, acetate starch, sulfamate starch, phosphatestarch, nitrogen modified starch, starch crosslinked with aldehydes,epichlorohydrin, borates, and phosphates, and starches grafted withacrylonitrile, acrylamide, acrylic acid, methacrylic acid, maleicanhydride, or styrene. Preferred among the modified starches arehydroxypropyl and carboxymethyl starches. While granule size of any ofthe above starch particles is not critical, commercially available sizesbeing suitable, a preferred range of dry particle sizes would be fromabout 5 μm to about 150 μm.

The particulate mica employed in the invention is a matter of choice. Asused herein, the term "mica" refers generally to natural and syntheticsilicate compositions of varying chemical composition characterized bybeing cleavable into thin sheets or plates that are flexible andelastic. Suitable micas include muscovite, phlogophite, biotite,zinnwadite, and pegmatite. As indicated, fine particulate mica isrequired. Preferably, the median particle size of the mica is smallerthan about 50 μm, most preferably below 32 μm.

If an additional inorganic solid or solids (i.e., additional anddifferent from mica) are employed along with the primary components, themedian particle size thereof will also be, as indicated, suitably small,generally in the same range as the mica particles, and preferably themedian particle size will be below about 50 μm. Preferred finely dividedinorganic solids include those of silica, limestone (CaCO₃), rock salt,alumina, talc, and kaolin.

The ratio of starch composition to mica, by weight, will range fromabout 11:1 to about 1:14, preferably from about 5:1 to about 1:7. Ifadditional finely divided inorganic solid or solids are employed, theinorganic solid(s) may replace some starch or mica in the overall solidscontent of the compositions. The finely divided inorganic solid(s) willpreferably have a weight ratio of such solid (or solids) to the mica offrom about 1:1 to about 5:1, the weight ratio of the finely dividedsolids to the starch composition thus being from about 7:1 to about 7:3.

While not wishing to be bound by any theory of invention, it is believedthat the somewhat deformable starch particles will partially fill porethroats in the formation, the mica particles, which are platelets,filling the remainder of the throats or voids. It also appears that micaparticles are particularly effective in stopping spurt under high shearrate conditions because plate-like particles appear to spend most oftheir rotation period aligning with the flow in a shear flow which isparallel to the fracture surface. Mica particles having the highestaspect ratio, that is the ratio of the largest dimension (diameter) overthe smallest dimension (thickness), apparently align with the flow. Thisallows the mica particles to sit on the pore throats of the fracturesurface thereby covering them more effectively. Also, once the micaparticles are on the fracture face, the torque exerted on the micaparticles by the shear stress of the flowing fluid is smaller than thatexperienced by granular-shaped particles of the same diameter undersimilar high-shear conditions. The mica platelets, thus, appear to bemore difficult to remove from the surface and thereby offer better fluidloss properties.

The invention thus provides a bimodal pore filling mechanismcharacterized by a deformable particle having improved resistance to ahigh shear fracture fluid, along with small particles which can aid insealing the pores. In this sense, the ratios of mica to starch mentionedare critical since, at ratios of mica to starch significantly belowthose designated, the voids in the fracture faces cannot be fullysealed, and, at insufficient ratios of starch to mica, the sealing ofthe larger pore throats may not be achieved. The additional finelydivided inorganic solids, when employed, will be used primarily in veryhigh permeability formations, e.g., greater than about 50 md, where theyare beneficial because of their rigidity.

Optionally, but preferably, the starch composition-mica mixture iscombined with a surfactant to aid dispersion of the dry starch-micamixture into the fracturing fluid. Useful surfactants include lower HLB(lipophilic) surfactants in the HLB range of about 1-11, with the HLBrange of 4-10 being preferred. Representative useful surfactants includesorbitan monooleate, polyoxyethylene sorbitan monooleate, ethoxylatedbutanol, and ethoxylated nonyl phenol, as well as various blends ofthese surfactants. The surfactants typically will be used at a level ofabout 0.1 to 10 percent by weight, and preferably about 0.5 to 5 percentby weight.

In practice, the additive components of the invention are normallydispersed, with the aid of the surfactant, into a suitable diluent orcarrier fluid. Suitable carrier fluids include low toxicity mineral oil,diesel fuel, kerosene, and mixtures thereof. Preferably, the carrier andadditive components will be combined in such manner that the starch willbe present in an amount of from about 2 percent to about 45 percent byweight, the mica being present in an amount of from about 8 percent toabout 25 percent by weight, all percentages based on the total weight ofthe carrier and components. If additional inorganic solid(s) arepresent, they will be present in an amount of from about 8 percent toabout 25 percent by weight, all percentages again based on the totalweight of the carrier and components. The combination additivecomposition plus carrier is then easily mixed with or dispersed into afracturing fluid.

The particular fracturing fluid employed with the additive components ofthe invention is largely a matter of choice and forms no part of thepresent invention. For example, fluids may comprise micellar solutionsor emulsions, uncrosslinked solutions of cellulose or guar, or may beborate, titanium, or zirconium crosslinked fluids, the particular fluidchosen being determined by such considerations as formation temperatureand concentration of proppant to be carried. As those skilled in the artwill be aware, however, the fracturing fluid and additive compositionsmust be compatible in the sense that they do not react with one anotheror otherwise deleteriously interfere with the designed functions ofeach. Preferably, the additive compositions of the invention areemployed with aqueous based fracturing fluids, although this is not arequirement. Particularly preferred are the types of fracturing fluidsdescribed by Nimerick, Crowe, McConnell, and Ainley in U.S. Pat. No.5,259,455, and those disclosed in U.S. Pat. No. 4,686,052.

As noted, the amount of additive components supplied in the fracturingfluid will be that amount sufficient or effective to provide the desiredfluid loss control. This concentration of additive will be varieddepending on the permeability and other characteristics of theparticular formation. Typically, from about 10 to 75 lbs of the additivecomponents of the invention per 1000 gallons fracturing fluid aredispersed in the fracturing fluid, with about 20 to 60 lbs of theadditive components per 1000 gallons of fracturing fluid representing apreferred range of addition. As indicated, the concentrations of each ofthe additive components in the fracturing fluid and the ratios thereforeare important if effective sealing of the pores is to be obtained. Ingeneral, the fracturing fluid will contain from about 2 lbs. to about 28lbs. of starch_composition and from about 2.5 lbs. to about 28 lbs. ofmica, per 1000 gallons of the fracturing fluid. If an additionalinorganic solid or solids are employed, the concentration of suchsolid(s) will range from about 4.0 lbs. to about 15.0 lbs. per 1000gallons of fracturing fluid, preferably from about 5.0 lbs. to about10.0 lbs. per 1000 gallons.

Following the practice of the invention, as the fracture is created inthe formation, the fluid loss control additive is deposited in the poresin the walls of the fracture to form a seal which controls the leak-offrate and confines the fracturing fluid to the fracture. Therefore, withthe same fluid volume, a longer fracture may be obtained. Again,contrary to what might be expected, experiments indicate that use oflower viscosity fracturing fluids containing the additive components ofthe invention gives better fluid loss control than when more viscousfluids are employed. This appears to be explained by the fact that theability of a particle to reach the porous fracture face depends on adrag force that the fluid moving toward the fracture face exerts on theparticle. This drag force is proportional to the leak off rate beforethe particles reach the fracture face, which increases with decreasingviscosity of the fluid and increasing rock permeability. This feature isused to particular advantage in a pre-fracturing fluid sacrificial fluidstage of a fracturing method which will be more fully described andillustrated by examples presented hereinafter.

In order to determine the fluid loss control properties of compositionsaccording to the invention, the following experiments were conducted.The experiments were carried out in dynamic fluid loss cells which weremodifications of the unit describe by Roodhart, L. P., SPEJ, (October1985), pp. 629-636. In the modified cells, dynamic fluid lossmeasurements were made while the test fluid flowed in slot geometry, acircular area in only one of the slot walls being porous. In each case,the surface area (4.97 cm²)and the length (2.54 cm) of the core employedwere the same. The width of the slot was the same as the diameter of thecore. The variables for each run were thus temperature, pressure, coretype and permeability, and shear rate.

In the experiments, aqueous fracturing fluids of the guar containingtype were prepared containing starch composition and mica, or starch,mica, and silica flour, in the proportions hereinafter indicated. Thestarch, mica, and silica, if present, were first slurried with a smallquantity of diesel No. 2, organophilic clay, and surfactant, for ease ofdispersion in the fracturing fluid. Each fracturing fluid containedtypical additives commonly present in such fluids, such as antifoam,bactericide, friction reducer, and delay agent. In the tables of resultsfor each experiment, to demonstrate the importance of the additivecomponents of the invention, comparisons are made, however, only withruns of identical or analogous fracturing fluid not containing theadditive components, under the same or substantially similar testconditions, the only significant differences for the "control" runsbeing the absence of the organophilic clay and surfactant, and somewhatlower content of Diesel No. 2.

In all cases, the shear rate was varied, as follows:

Between 0 sec. to 43 sec. - - - 380s⁻¹

Between 43 sec. to 6 min. - - - 304s⁻¹

Between 6 min. to 16 min. - - - 190s⁻¹

Between 16 min. to 26 min. - - - 133s⁻¹

Between 26 min. to 45 min. - - - 114s⁻¹

The results of the experiments, with relevant variables, are as follows:

I

In these runs, a Berea sandstone core having a specific permeability of1.90 was employed, and the temperature was 150° F. Column A lists thecomponents of the "control" fracturing fluid composition, while Column Blists those of the invention.

    ______________________________________    A                 B    Name     Concentration                          Name      Concentration    ______________________________________    Guar     25.000 lb/1000 gal.                          Guar      25.000 lb/1000 gal.    KCl      167.000 lb/1000 gal.                          KCl       167.000 lb/1000 gal.    Antifoam 0.250 gal/1000 gal.                          Antifoam  0.250 gal/1000 gal.    Bactericide             0.500 gal/1000 gal.                          Bactericide                                    0.500 gal/1000 gal.    Friction              Friction    Reducer  1.000 gal/1000 gal.                          Reducer   1.000 gal/1000 gal.    Boric Acid             5.000 lb/1000 gal.                          Boric Acid                                    5.000 lb/1000 gal.    Caustic Soda             10.000 lb/1000 gal.                          Caustic Soda                                    10.000 lb/1000 gal.    Delay Agent             20.00 lb/1000 gal.                          Delay Agent                                    20.00 lb/1000 gal.    Diesel No. 2             4.400 lb/1000 gal.                          Diesel No. 2                                    7.85 lb/1000 gal.                          Potato Starch                                    25.000 lb/1000 gal.                          Mica      5.000 lb/1000 gal.                          Clay      0.54 lb/1000 gal.                          Surfactant                                    0.150 lb/1000 gal.    ______________________________________

Fluid loss amounts (total), in milliliters, after the times indicated,were as follows:

    ______________________________________    A                      B    minutes   ml.          minutes ml.    ______________________________________    0.0       0            0.0     0    0.9       1.3          0.9     1.1    1.5       1.5          1.5     1.2    9.1       3.3          9.1     2.8    30.1      5.4          30.1    4.6    ______________________________________

Accordingly, at relatively low specific permeability, the inventioncomposition exhibits improved fluid loss control.

II

In this set, the Berea sandstone "control" core had a specificpermeability of 2.08, while the core used with the composition of theinvention had a specific permeability of 2.03. The temperature employedwas 250° F., and the amount of guar was increased to 30 lbs/1000gallons. All other parameters were the same as Run I.

Fluid loss amounts (total), in milliliters, were, as follows:

    ______________________________________    A                      B    minutes   ml.          minutes ml.    ______________________________________    0.0       0            0.0     0    0.9       1.0          0.9     0.5    1.5       1.2          1.5     0.9    9.1       3.3          9.2     2.5    30.2      6.1          30.1    4.6    ______________________________________

III

In this run, the Berea sandstone "control" core had a specificpermeability of 9.85, while the core used with the composition of theinvention had a specific permeability of 10.11. All other parameterswere the same as Run I. Fluid loss amounts (total), in milliliters,were, as follows:

    ______________________________________    A                      B    minutes   ml.          minutes ml.    ______________________________________    0.0       0            0.0     0    0.9       2.3          0.9     1.2    1.5       2.3          1.5     1.1    9.1       3.7          9.2     2.3    30.1      5.5          30.1    4.5    ______________________________________

IV

The variables of this set correspond to those of run II, except that theBerea sandstone "control" core had a specific permeability of 45.86 andthe core used with the composition of the invention had a specificpermeability of 49.28. Fluid loss amounts (total), in milliliters, were,as follows:

    ______________________________________    A                      B    minutes   ml.          minutes ml.    ______________________________________    0.0       0            0.0     0    0.9       1.9          0.9     1.1    1.5       2.3          1.5     1.3    9.1       4.6          9.2     3.1    30.2      7.6          30.1    5.7    ______________________________________

In these runs, the "control" core was a Berea sandstone having aspecific permeability of 200.42, the core used with the composition ofthe invention had a specific permeability of 199.60, and temperature was150° F. Column A sets forth the base fracturing fluid composition"control" while Column B defines the invention fluid.

    ______________________________________    A                 B    Name     Concentration                          Name      Concentration    ______________________________________    Guar     25.000 lb/1000 gal.                          Guar       25.000 lb/1000 gal.    KCl      167.000 lb/1000 gal.                          KCl       167.000 lb/1000 gal.    Antifoam 0.250 gal/1000 gal.                          Antifoam  0.250 gal/1000 gal.    Bactericide             0.500 gal/1000 gal.                          Bactericide                                    0.500 gal/1000 gal.    Friction              Friction    Reducer  1.000 gal/1000 gal.                          Reducer   1.000 gal/1000 gal.    Boric Acid             5.000 lb/1000 gal.                          Boric Acid                                    5.000 lb/1000 gal.    Caustic Soda             10.000 lb/1000 gal.                          Caustic Soda                                    10.000 lb/1000 gal.    Delay Agent             20.00 lb/1000 gal.                          Delay Agent                                    20.00 lb/1000 gal.    Diesel No. 2             4.400 lb/1000 gal.                          Diesel No. 2                                    7.850 lb/1000 gal.                          Potato Starch                                    2.000 lb/1000 gal.                          Silica    14.000 lb/1000 gal.                          Mica      14.000 lb/1000 gal.                          Clay      0.600 lb/1000 gal.                          Surfactant                                    0.150 lb/1000 gal.    ______________________________________

Fluid loss amounts (total), in milliliters, after the times indicated,were as follows:

    ______________________________________    A                      B    minutes   ml.          minutes ml.    ______________________________________    0.0       0            0.0     0    0.9       7.8          0.9     2.8    1.5       8.1          1.5     3.0    9.2       9.4          9.1     4.2    30.1      11.7         30.2    5.6    ______________________________________

VI

In this run, the Berea sandstone "control" core had a specificpermeability of 407.13, while the core used with the composition of theinvention had a specific permeability of 404.93. All other parameterswere the same as Run V. Fluid loss amounts (total), in milliliters,were, as follows:

    ______________________________________    A                      B    minutes   ml.          minutes ml.    ______________________________________    0.0       0            0.0     0    0.9       37.0         0.9     7.8    1.5       39.0         1.5     8.0    9.2       39.8         9.1     8.8    30.1      40.6         30.2    9.8    ______________________________________

In sum, the tests indicate good fluid loss control capability at highshear rates and across a wide spectrum of specific permeability andtemperature.

Fluid loss in the fracturing of high permeability (greater than about 50md) formations may also be addressed, in accordance with another aspectof this invention, through the use of a preliminary sacrificialconditioning stage prior to the pumping of the fracturing treatment. Inthis regard, a fluid loss additive is suspended in a relatively lowviscosity fluid for example, having a viscosity less than 150 centipoiseat 510 sec⁻¹, and pumped into the high permeability formation atfracturing pressures prior to pumping the "normal" pad fluid of afracturing treatment. As the fracture is created and is advancingradially outwardly from the borehole, there is a large amount of fluidleak off that occurs from the sacrificial conditioning fluid. This leakoff, in turn, causes the fluid loss additive suspended therein to bequickly and easily deposited in the pores on the newly exposed fracturefaces of the high permeability formation. The fluid leak off is quicklyarrested by the deposit of the fluid loss additive thereby retaining thehigh pressures necessary to extend the fracture and also to present a"conditioned" face of known, low permeability to the following stages ofthe fracturing treatment. The creation of the conditioned, known, lowpermeability fracture face allow the treatment designer to accuratelypredict when the desired tip screen out will occur so that the treatmentcan be optimized for proper fracture geometry and proper use andplacement of the desired amount of fracturing fluid and proppant.

In accordance with the invention, a low viscosity carrier fluid for thefluid loss additive is provided. Any low viscosity fluid can be used solong as it will suspend the required amounts of fluid loss additive, buta linear or crosslinked guar or cellulose gel utilizing 10 to 20 poundsof gel per one thousands gallons of carrier fluid are preferred fortheir ease of mixing, ready availability and low cost. While the starchand mica fluid loss additive of the invention is the preferred fluidloss additive in this preliminary sacrificial conditioning fluid, manyother fluid loss additives can be used in this process in accordancewith the invention. Thus, fluid loss additives comprising silica flour,mica, natural and synthetic hydrocarbon resins and starches such asnatural, pregelatinized, modified, and the like and combinations thereofmay be used in accordance with the invention.

The following examples will illustrate the use of a preliminarysacrificial formation conditioning stage ahead of the fracturing fluidin high permeability formations in order to achieve tip screen out atthe desired point in the pumping schedule.

EXAMPLE 1

For comparison, two fracturing operations were simulated for the sameformation comprising sandstone having a permeability of 79 md. The firstfracturing operation was initiated with three thousand gallons of apreliminary sacrificial conditioning stage in accordance with theinvention comprising 30 lbs of a mixture of starch and mica per thousandgallons of fluid were suspended in a borate cross-linked guar fluidcontaining 20 lbs of guar per thousand gallons thousand gallons offluid. This was followed by another 3200 gallons of pad and five rampedproppant stages containing 0, 2, 4, 8, 13 and 18 pounds of proppant perthousand gallons of fluid, respectively, suspended in a 30 lb perthousand gallons borate cross-linked gel. This treatment achieved apropped fracture half-length of 92 feet with tip screen out. A secondtreatment pumped with the same proppant ramping schedule but without thepreliminary sacrificial conditioning stage of the invention and usingonly a 30 lb crosslinked guar gel screened out at 36 feet.

EXAMPLE 2

Simulations were run to show the results of using the preliminarysacrificial conditioning stage with a (low) estimated formationpermeability, but which, in reality, included a zone having a muchhigher actual formation permeability, to determine the fracture lengthas compared with the same simulations run without the preliminarysacrificial conditioning stage. All pad and fraturing stages wereperformed with 25 lb per 1000 gallons borate-crosslinked guar gel. Thepreliminary sacrificial conditioning stage comprised 30 pounds of amixture of starch and mica per 1000 gallons of a 20 pound per 1000gallon borate-crosslinked guar gel fluid. In a design simulationassuming a formation permeability of 175-180 md, the fracture length was38 feet when the preliminary sacrificial conditioning stage was employedas compared with 16 feet when no preliminary sacrificial conditioningstage was used. Considering the true formation parameters including azone having an actual permeability of 490 md, the use of the preliminarysacrificial conditioning stage permitted a fracture length of 24 feetwhereas immediate sand out caused there to be no development of fracturelength whatsoever without the use of the preliminary sacrificialconditioning stage.

EXAMPLE 3

In a similar comparison as in Example 1, two fracturing operations weresimulated for the same formation comprising sandstone having apermeability of 175 md. The first fracturing operation was initiatedwith 14,000 gallons of a preliminary sacrificial conditioning stage inaccordance with the invention. Thus, 15 lbs of silica flour per thousandgallons of fluid were suspended in a proprietary micellar solutioncontaining 3 gallons of a viscoelastic surfactant per thousand gallonsof fluid. This was followed by five ramped proppant stages containing0,1, 4, 7, 10, 13, 16 and 18 pounds of proppant per thousand gallons offluid, respectively, suspended in micellar solution containing 5 gallonsof the proprietary viscoelastic surfactant per thousand gallons ofproprietary micellar fluid. This treatment achieved a propped fracturehalf-length of 72 feet with tip screen out. A second treatment pumpedwith the same proppant ramping schedule but without the preliminarysacrificial conditioning stage of the invention and using only the 5gallon per thousand gallons of the same proprietary micellar fluidscreened out at 47 feet propped half-length fracture.

It can be clearly seen that through the use of a preliminary sacrificialconditioning stage pumped prior to the normal fracturing fluid stages,longer fracture lengths are achieved than if only normal fluids areused. In high permeability formations, the use of the preliminarysacrificial conditioning stage of the present invention permits thedevelopment of longer fratures, even when the permeability of theformation would prohibit fracture formation without using a moreconcentrated, viscous and, thus, more damaging gel for the fracturingprocess.

While the invention has been illustrated in the more limited aspects ofpreferred embodiments thereof, other embodiments have been suggested andstill others will occur to those skilled in the art upon a reading andunderstanding of the foregoing specification. It is intended that allsuch embodiments be included within the scope of this invention aslimited only by the appended claims.

Having thus described our invention, we claim:
 1. A method of fracturinga subterranean formation comprising providing a preliminary sacrificialconditioning stage solution comprising a suspension of a fluid lossadditive;pumping the preliminary sacrificial conditioning stage solutioninto a subterranean formation at a pressure sufficient to fracture theformation and fracturing the formation and depositing fluid lossadditive in the fracture formed; pumping a pad fluid into the fractureformed at a pressure sufficient to fracture the formation and extendingsaid fracture; providing a carrier fluid with a proppant suspendedtherein, and pumping carrier fluid containing suspended proppant intothe fracture.
 2. The method of claim 1 in which fluid leak off into theformation occurs with the deposition of the fluid loss additive, and thecarrier fluid containing suspended proppant pumped into the fracture ispumped at a pressure sufficient to fracture the formation and thefracture is extended and proppant is deposited in said formation.
 3. Themethod of claim 1 in which the preliminary sacrificial conditioningstage solution comprises a low viscosity fluid, the formation fracturedis a high permeability formation, and the fluid loss additive isselected from starch, mica, silica flour, organic resins, and mixturesthereof.
 4. The method of claim 2 in which the preliminary sacrificialconditioning stage solution comprises a low viscosity fluid, theformation fractured is a high permeability formation, and the fluid lossadditive is selected from starch, mica, silica flour, organic resins,and mixtures thereof.
 5. The method of claim 1 in which the preliminarysacrificial conditioning stage solution comprises a suspension formed bycombining components selected from starch, mica, silica flour, organicresins, and mixtures thereof, in a low viscosity fluid.
 6. The method ofclaim 2 in which the preliminary sacrificial conditioning stage solutioncomprises a suspension formed by combining components selected fromstarch, mica, silica flour, organic resins, and mixtures thereof, in alow viscosity fluid.
 7. The method of claim 1 in which the fluid lossadditive comprises a mixture of starch and mica, the weight ratio ofstarch to mica being from about 11:1 to about 1:14.
 8. The method ofclaim 1 in which the preliminary sacrificial conditioning stage solutioncomprises a suspension formed by combining starch and mica in a lowviscosity fluid, the weight ratio of starch to mica being from about11:1 to about 1:14.
 9. The method of claim 2 in which the preliminarysacrificial conditioning stage solution comprises a suspension formed bycombining starch and mica in a low viscosity fluid, the weight ratio ofstarch to mica being from about 11:1 to about 1:14.
 10. The method ofclaim 1 in which the preliminary sacrificial conditioning stage solutioncomprises a finely divided inorganic solid or solids.
 11. The method ofclaim 2 in which the preliminary sacrificial conditioning stage solutioncomprises a finely divided inorganic solid or solids.
 12. The method ofclaim 7 in which the preliminary sacrificial conditioning stage solutionfurther comprises a finely divided inorganic solid or solids.
 13. Themethod of claim 8 in which the preliminary sacrificial conditioningstage solution further comprises a finely divided inorganic solid orsolids.
 14. The method of claim 9 in which the preliminary sacrificialconditioning stage solution further comprises a finely divided inorganicsolid or solids.
 15. A method of conditioning a fracture face of asubterranean formation comprising providing a preliminary sacrificialconditioning stage solution comprising a suspension of a fluid lossadditive;pumping the preliminary sacrificial conditioning stage solutioninto a subterranean formation at a pressure sufficient to fracture theformation and fracturing the formation and depositing fluid lossadditive in the fracture formed and conditioning a face of saidfracture; pumping a pad fluid into the fracture formed at a pressuresufficient to fracture the formation and extending the fracture;providing a carrier fluid with proppant suspended therein, and pumpingcarrier fluid containing suspended proppant into the fracture.
 16. Themethod of claim 15 in which fluid leak off into the formation occurswith the deposition of the fluid loss additive, and the carrier fluidcontaining suspended proppant pumped into the fracture is pumped at apressure sufficient to fracture the formation and the fracture isextended and proppant is deposited in said formation.
 17. The method ofclaim 15 in which the preliminary sacrificial conditioning stagesolution comprises a low viscosity fluid, the formation fractured is ahigh permeability formation, and the fluid loss additive is selectedfrom starch, mica, silica flour, organic resins, and mixtures thereof.18. The method of claim 16 in which the preliminary sacrificialconditioning stage solution comprises a low viscosity fluid, theformation fractured is a high permeability formation, and the fluid lossadditive is selected from starch, mica, silica flour, organic resins,and mixtures thereof.
 19. The method of claim 16 in which thepreliminary sacrificial conditioning stage solution comprises asuspension formed by combining components selected from starch, mica,silica flour, organic resins, and mixtures thereof, in a low viscosityfluid.
 20. The method of claim 16 in which the fluid loss additivecomprises a mixture of starch and mica, the weight ratio of starch tomica being from about 11:1 to about 1:14.
 21. The method of claim 16 inwhich the preliminary sacrificial conditioning stage solution comprisesa suspension formed by combining starch and mica in a low viscosityfluid, the weight ratio of starch to mica being from about 11:1 to about1:14.
 22. The method of claim 20 in which the preliminary sacrificialconditioning stage solution further comprises a finely divided inorganicsolid or solids.
 23. The method of claim 21 in which the preliminarysacrificial conditioning stage solution further comprises a finelydivided inorganic solid or solids.
 24. The method of claim 16 in whichthe conditioning produces an apparent low permeability.
 25. The methodof claim 21 in which the conditioning produces an apparent lowpermeability.
 26. A method of fracturing a high permeabilitysubterranean formation comprising providing a preliminary sacrificialconditioning stage solution comprising a suspension of a fluid lossadditive in a low viscosity fluid; pumping the preliminary sacrificialconditioning stage solution into a high permeability subterraneanformation at a pressure sufficient to fracture the formation andfracturing the formation and depositing fluid loss additive in thefracture formed, with leak off of fluid into the formation;providing acarrier fluid with proppant suspended therein, and pumping carrier fluidcontaining suspended proppant into the formation at a pressuresufficient to fracture the formation and extending said fracture anddepositing proppant in said formation.
 27. The method of claim 26 inwhich the fluid loss additive is selected from starch, mica, silicaflour, organic resins, and mixtures thereof.
 28. The method of claim 26in which the preliminary sacrificial conditioning stage solutioncomprises a suspension formed by combining components selected fromstarch, mica, silica flour, organic resins, and mixtures thereof, in alow viscosity fluid.
 29. The method of claim 26 in which the fluid lossadditive comprises a mixture of starch and mica, the weight ratio ofstarch to mica being from about 11:1 to about 1:14.
 30. The method ofclaim 26 in which the preliminary sacrificial conditioning stagesolution comprises a suspension formed by combining starch and mica in alow viscosity fluid, the weight ratio of starch to mica being from about11:1 to about 1:14.
 31. The method of claim 28 in which the preliminarysacrificial conditioning stage solution further comprises a finelydivided inorganic solid or solids.
 32. The method of claim 29 in whichthe preliminary sacrificial conditioning stage solution furthercomprises a finely divided inorganic solid or solids.
 33. The method ofclaim 30 in which the preliminary sacrificial conditioning stagesolution further comprises a finely divided inorganic solid or solids.34. The method of claim 2 in which the preliminary sacrificialconditioning stage solution has a viscosity less than 150 centipoise at510 sec⁻¹.
 35. The method of claim 7 in which the preliminarysacrificial conditioning stage solution has a viscosity less than 150centipoise at 510 sec⁻¹.
 36. The method of claim 9 in which thepreliminary sacrificial conditioning stage solution has a viscosity lessthan 150 centipoise at 510 sec⁻¹.
 37. The method of claim 20 in whichthe preliminary sacrificial conditioning stage solution has a viscosityless than 150 centipoise at 510 sec⁻¹.
 38. The method of claim 21 inwhich the preliminary sacrificial conditioning stage solution has aviscosity less than 150 centipoise at 510 sec⁻¹.